There is a fundamental gap in our understanding of how unique aspects of host physiology influences infection susceptibility, particularly in high-ris individuals who carry a disproportionate burden of morbidity and mortality. Though primarily a hematological disorder, one of the most frequent complications of sickle cell disease (SCD) is severe bacterial infection, particularly with Streptococcus pneumoniae (pneumococcus), which manifests as a fulminant sepsis. This host is 400 times more likely to experience lethal pneumococcal infection than healthy hosts. Despite both vaccination and penicillin prophylaxis, pneumococcal colonization and invasive pneumococcal disease persists in SCD patients. The long-term goal is to understand the bacterial and host factors that predispose SCD patients to greater infection risk and to leverage this knowledge to develop novel therapies tailored to this high-risk group. The objective in this particular application is the application of both genetic tools, namely transposon mutagenesis and sequencing, and the SCD murine system to precisely define pneumococcal pathogenesis in the context of SCD. The central hypothesis is that pneumococcal genetics can be exploited to more precisely define the selective pressures encountered in an altered host, reflecting the unique host physiology that drives heightened infection risk. Guided by preliminary data, this hypothesis will be tested by pursuing three specific aims: 1) Define the pneumococcal genes under differential selective pressure during colonization and pneumonia in SCD, 2) Ascertain the mechanisms underlying the retention of invasiveness by both atypical capsular types and altered core genome pneumococcal variants in SCD, and 3) Establish the metabolic factors in the SCD host rendering the host more permissive to invasive pneumococcal disease. These studies will close a significant gap in current knowledge by leveraging data from extensive genomic analysis of pneumococcal isolates from SCD patients and the capacity to model both infection and the contribution of genes to pathogenesis using Tn-seq, allowing a unique opportunity to investigate aspects of pneumococcal disease in the context of SCD in a clinically relevant setting. The research proposed enables the application of recently developed genetic tools to address fundamental questions about how invasive pneumococcal disease differs between those with SCD and their healthy peers from both the bacterial and host perspectives. Understanding the differences of host-pathogen interactions in the context of SCD will establish that this population requires distinct vaccine strategies and therapeutics to both prevent and treat bacterial infections and what these modified strategies should encompass. These results will have an important positive impact on human health by increasing understanding of invasive pneumococcal disease in the SCD population, revealing the factors underlying the increased risk of fatal infection despite current prophylaxis, and identifying potentially targetable pathways to mitigate the severity of infection.